1. Field of the Invention
This invention relates to catalysts for purifying exhaust and waste gases by the removal therefrom of nitrogen oxides. Particularly, it relates to catalysts for purifying exhaust and waste gases emanating from boilers and other similar stationary combustors at thermal power plants, iron foundries and other various plants by the removal of nitrogen oxides (mainly NO and NO.sub.2 ; hereinafter referred to as "NO.sub.x "). More particularly, it relates to catalysts which, in the treatment of an exhaust gas containing NO.sub.x and sulfur oxides (containing mainly sulfur dioxide; hereinafter referred to as "SO.sub.x ") with ammonia added thereto as a reducing agent, enable NO.sub.x to be efficiently decomposed into harmless nitrogen and water and, at the same time, substantially curb the oxidation of sulfur dioxide into sulfur trioxide. Moreover, these catalysts enjoy high durability.
2. Description of Prior Art
Heretofore, for the removal of NO.sub.x from the exhaust gas, there have been developed an adsorption method, an absorption method and a catalytic reduction method. Among other methods, the catalytic reduction method particularly suits the treatment of a large volume of exhaust gas. Among other versions of the catalytic reduction method, the version which effects catalytic reduction of NO.sub.x selectively to N.sub.2 by the addition of ammonia as a reducing agent has been reported to excel in performance. The question encountered by this version is whether or not there can be obtained a catalyst which befits the composition and behavior of the particular exhaust gas to be treated.
The requirements to be fulfilled by the catalyst usable for this method are firstly that the catalyst should be insusceptible to oxygen, SO.sub.x, carbon dioxide, carbon monoxide, steam, halides, hydrocarbons, etc. which coexist in the exhaust gas, secondly that it should produce ample performance at high space velocity in a wide range of temperatures, thirdly that it should not be deprived of activity by the deposition thereon of soot which accompanies the exhaust gas and contains heavy metals such as vanadium, nickel and iron and alkali metals such as sodium and potassium, and fourthly that it should be incapable of substantially oxidizing sulfur dioxide in the exhaust gas into sulfur trioxide. The sulfur dioxide contained in the exhaust gas is normally oxidized into sulfur trioxide on a catalyst. As the sulfur trioxide accumulates on the catalyst, it deprives the catalyst of its NO.sub.x removal activity or reacts with ammonia serving as a reducing agent to give rise to ammonium sulfate and ammonium hydrogen sulfate. When these ammonium salts accumulate in various devices such as a smoke stack or a heat exchanger, they eventually come to impede smooth operation of such devices. Thus, the oxidation of the sulfur dioxide into sulfur trioxide is desired to be curbed by all means. The catalyst for NO.sub.x removal from the exhaust gas is required to possess properties enough to fulfil the four requirements described above.
Heretofore, as a catalyst for NO.sub.x removal, an iron-titanium oxide type catalyst containing not more than 8% by weight of iron as FeO based on the total amount of the catalyst which is obtained by treating a high-titanium slag containing 2 to 8% by weight of iron as FeO with sulfuric acid under application of heat, optionally filtering the resultant solution and discarding the filtrate, neutralizing the solids resulting from the treatment of high-titanium slag with sulfuric acid, drying the neutralized solids and calcinating (U.S. Pat. No. 4,036,785) has been known to the art. This catalyst, however, has a disadvantage that the NO.sub.x removal efficiency is low and the service life is insufficient.
A catalyst which has as its catalytically active component a mixed composition obtained by mixing titanium dioxide with at least one member selected as a second component from the group consisting of the oxides of copper, nickel and cobalt at an atomic ratio in the range of 0.01 to 1 based on titanium has been also known to the art (U.S. Pat. No. 4,085,193). Again this catalyst has a disadvantage that the activity is low and the service life is still insufficient.
We have already disclosed in U.S. Pat. No. 4,221,768 a catalyst which has high NO.sub.x removal ability, possesses satisfactory durability and minimally oxidizes sulfur dioxide. We have learnt, however, that the treatment of an exhaust gas from a boiler burning coal or an exhaust gas from a boiler burning heavy oil of high sulfur content necessitates use of a catalyst which is much less active in the oxidation of sulfur dioxide and more effective in NO.sub.x removal from the exhaust gas.
The results of our study reveal that vanadium oxide present in the catalyst brings about an outstanding activity in NO.sub.x removal from the exhaust gas and, in the meantime, constitutes itself a major cause for enhancing the oxidation of sulfur dioxide. The ability of this catalyst to oxidize sulfur dioxide can be lowered by decreasing the vanadium oxide content thereof, though inevitably with a proportional sacrifice of the ability to remove NO.sub.x. It has, thus, become necessary to study various catalyst compositions in search of a catalyst composition which is capable of providing high NO.sub.x removal ability at a lower vanadium oxide content.
An object of this invention, therefore, is to provide a novel catalyst for purifying an exhaust gas by the removal of nitrogen oxides.
Another object of this invention is to provide a catalyst which, in the treatment of an exhaust gas containing both NO.sub.x and SO.sub.x with ammonia added thereto as a reducing agent, enables NO.sub.x to be decomposed effeiciently into harmless nitrogen and water and, at the same time, substantially curbs the oxidation of sulfur dioxide into sulfur trioxide while exhibiting outstanding durability.